1. Field of the Invention
The invention is directed to a method and apparatus for dynamically encoding a pantograph on an original document which indicates invalidity or inauthenticity when the document is copied. More specifically, the present invention relates to a method and apparatus for devising a pantograph message in real time and as a function of the pantograph background without consuming substantial computer memory and without forming a watermark on the document.
2. Description of Related Art
Fraud associated with written or printed documents such as bank checks or secured notes is an old and well-known problem. Fraud includes alteration, counterfeiting and copying. One type of fraudulent check is one that has been altered to provide a higher render value than originally intended. Another common fraudulent practice is printing counterfeit checks that appear identical to the original checks. Measures for authenticating such documents are as common as the counterfeit industry.
Historically, watermarks have been used to combat the counterfeit problem. A watermark is a recognizable image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light on top of a dark background), caused by thickness variations in the paper. Watermarks vary greatly in their visibility. While some are obvious on casual inspection, others require some study to pick out. Various aids for visual inspection have been developed, such as a watermark fluid that wets the paper without damaging it.
Because watermarks are encoded on paper or other substrates, their application is expensive and time-consuming. Once the watermark has been detected and copied, the protection it affords is nullified and any inventoried watermark paper must be destroyed. Finally, changing the watermark is a time-consuming and expensive process.
Another approach for combating counterfeit is encoding the document with a pantograph. A pantograph is a printed message that is not readily distinguished on the original printed page, but becomes more clearly visible on a subsequent photocopy. Instead of being printed as fully filled-in (i.e., opaque) characters, the characters in the pantograph message are filled with a specific pixel formation. The background for the pantograph message is another pixel formation which is similar to the format pixel formation used for the characters in the message. The two formations have a similar appearance, so that they appear to blend together. A document with a pantograph message appears to have a shaded or textured background. The two patterns are designed and paired in such a way that they are more differentiated when photocopied due to individual pixels not being precisely reproduced. Thus, the pantograph message (for example, the words “void” or “copy”) is more visible on the copy. Encoding with a pantograph has advantages over watermarking. A copy of a watermarked document does not bear the watermark. Only the original does. One has to know the original is watermarked, to detect a copy. No such prior knowledge is required with a pantograph.
Conventional pantographs include providing special paper which has been pre-printed with a pantograph image. The paper tray in the printer is then loaded with the pre-printed paper which has the pantograph image. The advantage of this implementation is that the pantograph is composed of higher resolution patterns than a printer is capable of imaging. However, the pre-printed paper must be printed and inventoried, which can be expensive. As with watermarking, the pre-printed pantograph paper does not lend itself to quick and inexpensive changes.
Software applications have also been used in pantograph applications. The software is used to add a pantograph to the document as a bitmap image background. The bitmap occupies a significant part of the target page and may occupy several megabytes in addition to the document itself. The bitmap image increases the size of the file requiring a greater amount of time to transfer the document to the printer. Additionally, access to the files containing the pantograph image requires control to prevent misuse. If a different pantograph message is required, a different bitmap must be implemented.
Due to differences in copying techniques used by different copy machines, the appearance of any pantograph on copied pages can differ significantly from one copier to another. Single pattern pairs often fail to produce acceptable results on a wide range of copiers. It is desirable to create a pantograph background that produces acceptable results on a variety of monochrome and color copiers.
There is a need for an improved method and apparatus to dynamically encode security images (i.e., pantograph messages) on a document without significant cost or memory consumption.